1. Field of the Invention
The present invention relates to an optical information reading apparatus, and particularly to an optical information reading apparatus suitable for reading information on a recording medium with a high density.
2. Description of the Related Art
Recently, DVDs (digital video disks) having a recording capacity about 7 times as large as that of CDs (compact disks) have been widely used to meet the demand for an increased recording capacity. Increase in recording capacity means improvement of the recording density, which depends upon the number of recording pits formed on a recording medium (hereinafter referred to as disk). In DVDs, one way for increasing the recording density is to decrease the size of a recording pit, that is, reduce the diameter of the spot of a laser beam radiated on the disk.
The size of a micro spot radiated on the disk is proportional to the wavelength of a laser beam and is inversely proportional to the numerical aperture of an objective lens. In order to reduce the diameter of the spot, therefore, it is necessary to decrease the wavelength of the laser beam and/or to increase the numerical aperture of the objective lens. However, when the numerical aperture of the objective lens is increased, the converging state of a laser beam deteriorates due to coma aberration with respect to the inclination of the optical disk. Since coma aberration is proportional to the third power of the numerical aperture of the objective lens and to the thickness of a disk substrate, DVDs are designed to have a disk substrate thickness of 0.6 mm, which is half that of CDs.
Reading out information requires a tracking servo to trace the track formed on the disk, and a focusing servo to follow defocusing caused due to the vertical movement of the disk. In order to read information from a disk such as a write-once-read-many optical disk or a phase transition optical disk, which is formed with grooves and portions therebetween pre-wobbled, it is necessary to perform an accurate tracking servo operation, which is performed predominantly by a DPP (differential push-pull) method which reduces track offset caused due to the inclination of the disk. The DPP method uses two sub-beams in addition to a main beam from a semiconductor laser to perform the tracking servo operation, thus requiring a diffraction grating for generating sub-beams.
FIG. 7 is a block diagram showing the main section of a conventional optical information reading apparatus. The conventional optical information reading apparatus comprises a semiconductor laser 1B, a diffraction grating 20, a beam splitter 3, a collimating lens 4, a reflecting mirror 5, an actuator 9, and a photodetector 2. The actuator 9 comprises an objective lens 8 and a control circuit 21, and is moved by a driving means (not shown) relative to the surface of a disk (not shown) disposed above the objective lens 8. A laser beam is converged onto the surface of the disk (not shown) by the objective lens 8. The photodetector 2 is a well-known eight division photodetector with a signal processing circuit.
The above-described optical information reading apparatus operates as follows. A laser beam emitted from the semiconductor laser 1B and having a wavelength appropriate to a disk used is separated into a 0-th order laser beam and ± primary laser beams by the diffracting grating 20. The 0-th order laser beam and the ± primary laser beams travel via the beam splitter 3, the collimating lens 4 and the reflecting mirror 5, are incident on the objective lens 8 and converged thereby onto the disk (not shown). The 0-th order laser beam and the ± primary laser beams converged onto the disk (not shown) are reflected thereat, take the incoming path backward, and are made incident on the beam splitter 3. These reflected laser beams incident on the beam splitter 3 pass therethrough, are made incident on the photodetector 2, and separately converted thereby into electrical signals using a well-known method such that the 0-th order laser beam is converted into recorded information and focusing control signals while the ± primary laser beams are converted into a tracking control signal. The disk is controlled by a well-known control circuit, and the signals are read out by a well-known signal processing circuit (both circuits not shown).
In such a conventional optical information reading apparatus, the following problems have been involved. The laser beam emitted from the semiconductor laser 1B is separated into a 0-th order laser beam and ± primary light beams by the diffracting grating 20. However, the diffracting grating 20 has a pitch distance as small as 20 μm and therefore is difficult to produce, thereby making the production cost high. Furthermore, the wavelength of the semiconductor 1B must be matched with a high-density disk, making densification difficult.